spadeng,

PilotFighter and Dr1Driver gave the best answers.

I'll just ad that there is no one right prop for a given plane, just like there is no one right gear to drive your car in. It all depends on what you want to do and how badly you need to get maximum propulsion efficiency. A racer for example would use a different prop on a larger course than a smaller one. The plane will spend more time flying straight on a larger course, and more of the time turning on a smaller course. One pilot may enjoy short takeoffs and steep climbs, while another likes to run flat out straight and level.

Smaller pitch props are "climb" props. They give better pull at slow speed and better braking during glide. Larger pitch props are"speed" or "cruise" props. They give better pull at top speed, but climb and braking suffer. An in flight variable pitch prop is the best solution. For RC flying multi-pitch props are available, where the blades are shaped with a combination of pitches in a range. For example, instead of a 10x6 or a 10x8, you might find a 10x6-8. It won't climb or speed as well, but does a little better over all.

Notice I didn't mention diameter. That is because different operators may prefer to run the same engine at different rpms. One operator may be using a tuned exhaust and looking for the absolute screaming max power possible. Another operator may want to burn cheaper low nitro fuel, have a noise restriction to meet, or simply want to run softer so it lasts longer.

So you have to find the airspeed you want to have best prop efficiency at. Then decide what rpm you want your engine to spin at that speed. Then select the best prop for your choices.

Choose the pitch to match desired rpm with the desired speed, then choose diameter for engine load. It takes lots of experimentation to find the best prop for a specific application. Many experienced modelers have accumulated a large collection of test props to experiment with for each new plane.

Multiflyer